Recently WORM optical memory devices have experienced great evolution, providing recording of data with the possibility of its immediate reading. This feature—data recording in a real-time regime—is significant for various applications of optical recording in memory devices, especially for computer systems. For this field duplication of data is not so essential.
All WORM optical media of practical interest is based on photothermal principle of recording. The data on such media is recorded by scanning the recording layer with the focused laser beam. The laser power is absorbed by the active medium of the layer and transformed into thermal energy, causing its physical and chemical changes, which can be optically registered at reading.
Photochemical effects can also be used, i.e. optically detected changes in the state of medium, caused by direct interaction of photons with this medium. The efforts are made to use photosensitive medium for photochemical recording on WORM discs. Hence, until now there was no practical application for WORM discs with photon mechanism of recording. The reason can be the non-threshold nature of photochemical recording on the contrary to photothermal recording at the same laser for recording and reading (with different laser power). Therefore, the photochemical recording can not provide the necessary stability of medium characteristics at multiple reading.
According to the mechanisms of thermally induced effects, the photothermal recording on WORM optical medium with practical applications can be divided in two parts:    1. Ablative, providing optically registered geometric changes in the thin active layer during its melting, evaporation or chemical transformations, and    2. With phase change, which does not provide geometric changing of the active layer, otherwise changing its optical constants, that causes optical contrast, which is usually not high for these materials.
Among various types of medium for ablative recording, WORM optical discs with thin (10–100 nm) layers of organic dyes with or without dye-in-polymer are of special interest. Layers of organic dyes provide a range of sufficient advantages in comparison to metal or half-metal layers, used in WORM discs with ablative recording. Advantages are the following:    Dyes may have a stronger selective absorption on the recording laser wavelength.    Dye layers are more sensitive to the laser radiation because of their small thermal conductivity and low temperature of melting or decomposition. It provides a higher recording capacity.    Dye layers provide a higher stability at higher humidity.    Medium based on dye layers has better signal-to-noise ratio, because of the lack of noise, provided by amorphous layers.    Coating in the centrifuge makes the layers, that is more simple and cheap than vacuum deposition used for obtaining metal and half-metal layers on WORM discs.
The existing WORM optical discs based on organic dyes has a capacity up to 3.5 GB.
The WORM discs with one recording layer this optical memory capacity is the utmost at least for the diode laser with 780–830 nm wavelength.
Future capacity increase for WORM discs is possible only using three-dimensional optical memory carriers with multilayer data recording and fluorescent reading [A. S. Dvornikov, P. M. Rentzepis, Opt. Comms., v.136, pp. 1–6 (1997); B. Glushko, U.S. Provisional Patent Application, May 8, 1997, N 25457.].
Fluorescent reading offers a range of sufficient advantages in comparison to reading, based on changing the reflection ratio, even in single-layer systems.
One of the advantages is the reduced tolerance for the sizes of recorded pits in comparison to the existing WORM discs. I.e., changing the size on a 100 nm does not influence the reading from fluorescent disc, while it totally eliminates the signal from reflective discs.
Another advantage is the reduced sensitivity of fluorescent discs to changing the slope up to 1 grad that is absolutely intolerable for reflective discs.
Nevertheless, the basic advantage of fluorescent reading is the enhanced capacity of three-dimensional optical memory carriers, realized as multilayer discs.
Use of layers of organic dyes with ablative recording in such medium is not possible owing to the following reasons:    Reading is realized by laser beam, scanning the change of reflection in the pre-irradiated spots. In a multilayer system, this method causes a strong fall of reading quality, becoming dramatic for systems with over four active layers.    Heat change of the layer geometrical structure at recording, such as: burning out of holes, creation of bubbles, change of surface texture, etc. It is also unsuitable for multilayer medium, as it causes dispersion of the reading beam, hence abruptly lowering the level of fluorescent signal.    The dye concentration in the recording layer of the existing WORM discs is the utmost (up to 99%). In this case, the dye fluorescence is usually suppressed because of high concentration.
In the thin dye layers (10–100 nm) of the existing WORM discs the local heating of the medium at recording can reach 700° C. Such high temperature make it difficult to avoid changing the geometrical structure of the layer. Increase of the dye layer thickness up to 200 nm and more using polymer dye at preserving the surface concentration of dye leads to lowering the local heating temperature and allows to prevent the layer deformation. It also provides the appearance and growth of the dye fluorescence due to lowering the concentration suppression effect. However at all the same conditions the layer sensitivity to laser radiation is dramatically lowering, that leads to drop of recording speed and density.
Thus, all the known materials, used for single-layer optical WORM discs with reflective reading, as well as photothermal recording methods can not be used for multilayer optical WORM discs with fluorescent reading. Comparatively thick layers (200 nm and more) of fluorescent dye are likely not suitable for multilayer medium as well without use of special additives and ways of recording, increasing recording speed and density.